Have you tried using rs232 instead of SPI for the dosonchip? This thing is on my wishlist, and I'm curious to see how well it works. I think there's another vendor with a similar device, but I can't remember details (sorry).
One way to use SD/CF/etc is to simply use the SPI interface and use it as a chunk of flash memory. Of course, you won't have a filesystem left so you can't just plug it in to a cardreader and grab a data file (although a linux user should be able to use dd to pull the entire contents off easily).A trick I've heard about (but haven't tried) is to create a dummy file with junk in it, fighure out where that file starts on flash, then make sure you skip that many bytes (and don't write past the length of the file) when writing to the flash device. I don't particularly like the sound of this, but it does have the potential of being the cheapest in terms of interface hardware and software development.
Then again, you can always implement FAT in arduino and share your hard work with the rest of us.
I still have pc's with a RS232 interface. But no max232 or something like that to get the dosonchip voltages to rs232 levels. I tried to get it going with a software serial port on the arduino but it didn't work.
You could try the arduino's hardware UART (e.g. pin 0 and pin 1) - switch to external power (to disable the FTDI chip) and you can hook the dosonchip TTL level 232 to the arduino's TTL level 232.